1. Field of the Invention
This invention relates to a method of laser transformation hardening which is utilized in quenching surface of a work piece for improving the property thereof.
2. Discussion of Background
Generally speaking, the absorption coefficient of a metallic material as an object of laser beam machining, is small. Therefore, conventionally, it has been a big problem how to transfer energy of irradiated laser beam to a work piece efficiently, to thereby elevate temperature of the work piece.
This is not exceptional in laser transformation hardening. A method is adopted in which the laser beam is irradiated on the work piece, generally, after a pretreatment is performed such as painting of an absorber on the surface of the work piece, to improve the absorption coefficient of the laser beam on the surface of the work piece. However, in this method, steps of painting and removing the absorber are required.
The absorption coefficient of the laser beam depends on an angle of incidence of the laser beam with respect to the work piece. It is known that the absorption coefficient has its maximum value at Brewster angle inherent in respective material and the wave length of the laser beam.
This is called Brewster effect. It is possible to promote the absorption coefficient of the laser beam by utilizing this effect without applying a coating of the absorber on the surface of the work piece.
FIG. 5 shows the dependency of the absorption coefficient on the angle of incidence of a carbon dioxide laser beam with respect to a ferrous material having considerably smooth surface. The bold line shows the absorption coefficient of the carbon dioxide laser beam when the laser beam is incident on the material in P polarization, and the broken line shows the absorption coefficient when the laser beam is incident on the material in S polarization.
As shown in FIG. 5, the absorption coefficient in P polarization has its maximum value at around the angle of incidence of 85.degree., when the laser beam is incident on the material in P polarization. The maximum value is more than 10 times as much as the absorption coefficient when the laser beam is transmitted from right above.
FIG. 3 shows that a laser transformation hardening is performed on a ferrous material utilizing Brewster effect. FIG. 4 is a magnified diagram of the essential part in FIG. 3.
In FIGS. 3 and 4, a notation 1 designates a ferrous material test piece which is a work piece, 2, a laser beam, 3, a quench-hardened layer, .theta., an angle of incidence of the laser beam 2, and S, a horizontal plane of the work piece.
As shown in FIG. 4, the surface profile of the work piece which is observed microscopically, is a random irregularity. Therefore it is known by experiment that the angle of incidence .theta. which maximizes the absorption coefficient of the laser beam 2, is around 79.degree. which is a little lower than the theoretical value.
However, in the conventional method of laser transformation hardening, it is required to position the work piece to be inclined at almost in the neighborhood of 90.degree. with respect to the incident beam. Therefore the irradiated position of the laser beam is considerably deviated by a slight positional deviation of the work piece, which causes the positional deviation of the laser beam.
Furthermore, by the similar cause, the beam shape on the surface of the work piece is considerably changed by the slight deviation of the angle of inclination of the work piece, and the power density of the laser beam is also changed. Therefore it is quite difficult to maintain constant the condition of irradiation, and accordingly the stable laser beam machining can not be performed.